Network Working group DHC load balancing algorithm Oct 1999
Internet Draft Bernie Volz
Steve Gonczi
Process Software
Ted Lemon
Internet Engines, Inc.
Rob Stevens
Join Systems, Inc.
October 1999 Expires Apr 2000
DHC load balancing algorithm
<draft-ietf-dhc-loadb-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This draft proposes a method of algorithmic load balancing
that enables multiple, cooperating servers to decide which one
should service a client, without requiring participating servers
to exchange information.
It proposes a computable server selection mechanism for the
DHCP or DHC Failover Protocol.
In addition, it provides for the use of the same mechanism
to govern the target server selection of a forwarding agent
such as a BOOTP relay.
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1. Introduction
This protocol was originally devised to support a specific load
balancing optimization of the DHC Failover Protocol [FAILOVR].
The authors later realized that it could be used to optimize the
behavior of cooperating DHCP servers and the BOOTP relay agents that
forward packets to them.
This proposal makes it possible to set up each participating server to
accept a preconfigured (approximate) percentage of the client load.
This is done using a deterministic hashing algorithm, and
assumes that the hash will produce an even distribution of values
based on client load. Whether the distribution is in fact even for
any given set of clients is dependent on the clients, but the hash is
expected to produce reasonably evenly distributed output in all cases.
This algorithm could easily be applied to other protocols that have
similar characteristics and for which load balancing would be helpful.
2. Terminology
This section discusses both the generic requirements terminology
common to many IETF protocol specifications, and also terminology
introduced by this document.
2.1. Requirements terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
2.2. Load balancing terminology
This document introduces the following terms:
Hash Bucket Assignments, HBA
A configuration directive that assigns a set of hash bucket values to
a server participating in the load balancing scheme.
Server ID, SID
An identifier that can be used to designate one of the participating
servers in the context of another protocol implementing this
proposal. In the context of DHCP, this SHOULD be the IP address or
DNS name of the server.
Service Transaction, ST
A set of client-server exchanges that lead to a server providing or
denying some service to a client. Example: the DISCOVER/OFFER/
REQUEST/ACK message exchange between a DHCP server and client is a
service transaction. A service transaction may contain one or more
messages.
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Service Transaction ID, STID
An attribute of the individual client requests used for load
balancing. Deciding which attribute to use is entirely up to the
specific implementation.
3. Background and External Requirements
Because DHCP clients use UDP broadcast to contact DHCP servers, a client
DHCPDISCOVER message may be received by more than one server. All
servers receiving such a broadcast may respond to the client, letting
the client choose which server it will use.
When a BOOTP relay agent is used, it typically forwards or rebroadcasts
client broadcasts to all configured servers, so a similar inefficiency
is present.
The optimization described allows a server to be chosen for each
such transaction by performing a "serve" / "do not serve" computation.
A forwarding agent can perform the same computation to choose a
forwarding destination.
In either case, the choice of server can be computed, without the
participants having to negotiate who is to respond.
Each client request MUST have some hashable property that varies from
ST to ST, though it is not required that the attribute values should be
unique for each ST. The selected attribute MUST have the same value for
each message making up a multi-message ST.
The approach is probabilistic in nature, because it is nearly impossible
to foresee which client will request service next. For short periods
of time, the actual percentage of clients served by a given server
will likely deviate from the desired percentage. As the number of
requests grows, the actual percentage of the load being handled by
each server will approximate the configured percentage.
4. Overview
The specific implementation MUST choose an ST attribute that will
be used as the ST "key" for load balancing.
DHCP servers MUST use the Client Identifier option as the STID if it
is present. If no Client Identifier option is present, the hlen
field of the DHCP packet should be used as the length of the data to
be hashed, and the contents of the chaddr should be the data to be
hashed, except that in no event should more than sixteen bytes be
used.
Other implementations MAY choose other attribute(s) as their STID.
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The proposal maps the chosen STID into a hash value using the
function in section 6. The resulting hash value can then be used
to decide who should respond to the request, or who the forwarding
target should be.
The provided hash function generates hash values 0 to 255,
and yields a fairly even hash bucket distribution for random STIDs, and
also for STID sequences that have some pattern.
Resource allocation is accomplished by assigning specific hash
values to each participating server.
Each server is assigned ownership of one or more hash values,
and will only service requests where the STID hash matches one
of these values.
Any hash buckets not assigned to servers will result
in some client ST-s being entirely ignored. (In some scenarios,
this may be the desired outcome.) STID-s need not be unique, but
should have sufficient variety to exercise each hash value
assigned to any server.
5. Operation
5.1 Configuration
The configuration step consists of assigning hash values
to available servers. This is accomplished by providing one
or more Hash Bucket Assignments (HBAs). (These may come from
a configuration file, the Windows NT registry, EEPROM, etc.)
Alternatively, HBAs can be transmitted as messages,
encapsulated in messages of another protocol, for example using
e-mail, a DHC Failover Protocol option, or some other mechanism.
5.2 HBA intended for a participant server
When configuring one specific server, an HBA in the form of a
simple bit map of 32 octet values MAY be used. If the HBA is
represented in this form, the first octet in the HBA bitmap will
represent HBA values 0-7, the next byte values 8-15, and so on,
with the thirty-second octet representing values 248-255.
In each octet, the most significant bit in that octet represents
the largest HBA value represented in that octet.
So for example bit 7 of the first octet represents HBA value 7, and
bit 0 of the first octet represents HBA value 0.
Each bit of the HBA is associated with one possible hash
value. If a bit is set in the map, it means the recipient server
MUST service each client request, where the STID yields the
corresponding hash value.
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For example, if a server receives a HBA
with the following 32 octets:
buckets
FF FF FF FF FF FF FF FF ( 0 - 63 )
FF FF FF FF FF FF FF FF ( 64 - 127 )
00 00 00 00 00 00 00 00 ( 128 - 191 )
00 00 00 00 00 00 00 00 ( 192 - 255 )
then it MUST service any client requests where the STID
hashes into the bucket values of 0 through 127.
The above example is merely for illustration
purposes. An application implementing this algorithm is free
to choose a different format, as long as the server is informed
in some way of hash bucket values it owns.
5.3 HBA intended for a forwarder
When configuring a forwarding agent, (e.g.: BOOTP relay)
HBAs consisting of pairs of Server-ID / Hash Bucket values
MAY be used.
Here, the Server ID (SID) designates the server responsible for
the specified Hash Bucket. The forwarding agent
forwards each client request, where the STID yields the
specified hash value, to the server designated by the SID.
The Server ID may be any unique server attribute,
(E.g.: IP address, DNS name, etc) that is meaningful in the context of
the relay agent operation.
A forwarder may be configured to forward a packet to
more than one server. For example, a BOOTP relay could be
set up to split the load between 2 primary-backup server pairs,
running the DHC Failover Protocol [FAILOVR].
A possible configuration file for a forwarding agent
(e.g.: BOOTP relay) may look like this:
192.33.43.11 0 .. 24;
192.33.43.12 25 .. 55;
192.33.43.13 56 ..128;
192.33.43.14 129..255;
192.33.43.15 129..255;
The above configuration consists of 5 HBAs. The first HBA states:
"Any Client request, where the STID yields a hash value
0 to 24, will be forwarded to server 192.33.43.11".
Note that that HBA #4 and #5 instruct to forward the same requests
to both 192.33.43.142 and 192.33.43.14.
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The above example is merely for illustration purposes. An implementing
application is free to choose a different format, as long as the
forwarding agent is given a list of SIDs, with the set of hash bucket
values each server owns.
6. Hash function for load balancing
The following hash function is a c language implementation of the
algorithm known as "Pearson's hash". The Pearson's hash algorithm
was originally published in [PEARSON] To make this proposal work, all
interoperable implementations MUST use the same hash function.
/* A "mixing table" of 256 distinct values, in pseudo-random order. */
unsigned char loadb_mx_tbl[256] =
{
251, 175, 119, 215, 81, 14, 79, 191, 103, 49,
181, 143, 186, 157, 0, 232, 31, 32, 55, 60,
152, 58, 17, 237, 174, 70, 160, 144, 220, 90,
57, 223, 59, 3, 18, 140, 111, 166, 203, 196,
134, 243, 124, 95, 222, 179, 197, 65, 180, 48,
36, 15, 107, 46, 233, 130, 165, 30, 123, 161,
209, 23, 97, 16, 40, 91, 219, 61, 100, 10,
210, 109, 250, 127, 22, 138, 29, 108, 244, 67,
207, 9, 178, 204, 74, 98, 126, 249, 167, 116,
34, 77, 193, 200, 121, 5, 20, 113, 71, 35,
128, 13, 182, 94, 25, 226, 227, 199, 75, 27,
41, 245, 230, 224, 43, 225, 177, 26, 155, 150,
212, 142, 218, 115, 241, 73, 88, 105, 39, 114,
62, 255, 192, 201, 145, 214, 168, 158, 221, 148,
154, 122, 12, 84, 82, 163, 44, 139, 228, 236,
205, 242, 217, 11, 187, 146, 159, 64, 86, 239,
195, 42, 106, 198, 118, 112, 184, 172, 87, 2,
173, 117, 176, 229, 247, 253, 137, 185, 99, 164,
102, 147, 45, 66, 231, 52, 141, 211, 194, 206,
246, 238, 56, 110, 78, 248, 63, 240, 189, 93,
92, 51, 53, 183, 19, 171, 72, 50, 33, 104,
101, 69, 8, 252, 83, 120, 76, 135, 85, 54,
202, 125, 188, 213, 96, 235, 136, 208, 162, 129,
190, 132, 156, 38, 47, 1, 7, 254, 24, 4,
216, 131, 89, 21, 28, 133, 37, 153, 149, 80,
170, 68, 6, 169, 234, 151
};
unsigned char loadb_p_hash(unsigned char *key,/* The key to be hashed */
int len) /* Key length in bytes */
{
unsigned char hash = len;
int i;
for( i=len ; i > 0 ; )
hash = loadb_mx_tbl [ hash ^ key[ --i ] ];
return( hash );
}
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7. Security
This proposal in and by itself provides no security, nor does
it impact existing security.
Servers using this algorithm are responsible for ensuring that if
the contents of the HBA are transmitted over the network as part of
the process of configuring any server, that message be secured
against tampering, since tampering with the HBA could result in
denial of service for some or all clients.
8. References
[FAILOVR] Droms, R., Rabil, G., Dooley, M., Kapur, A., Gonczi, S.,
Volz, B., "DHCP Failover Protocol", Internet Draft
<draft-ietf-dhc-failover-05.txt>, June 1999.
[PEARSON] The Communications of the ACM Vol.33, No. 6 (June 1990),
pp. 677-680.
[RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC2131,
March 1997.
[RFC2219] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC-2219, March 1997.
9. Acknowledgements
Special thanks to Peter K. Pearson, the author of Pearson's hash
who has kindly granted his permission to use this algorithm,
free of any encumbrances.
This proposal stems from the original idea of hashing MAC addresses
to a single bit by Ted Lemon, during a Failover Protocol discussion
held at CISCO Systems in February, 1999.
Rob Stevens suggested the potential use of this algorithm for
purposes beyond the Failover Protocol.
Many thanks to Ralph Droms, Kim Kinnear, Mark Stapp, Glenn Waters,
Greg Rabil and Jack Wong for their comments during the ongoing discussions.
10. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
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assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works. However, this document itself
may not be modified in any way, such as by removing the copyright notice
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or references to the Internet Society or other Internet organizations,
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The limited permissions granted above are perpetual and will not be
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11. Author's information
Bernie Volz
Steve Gonczi
Process Software Corporation
959 Concord St.
Framingham, MA 01701
Phone: (508) 879-6994
EMail: volz@process.com
gonczi@process.com
Ted Lemon
Internet Engines, Inc.
950 Charter Street
Redwood City, CA 94063
Phone: (650) 779 6031
EMail: mellon@isc.org
Rob Stevens
Join Systems, Inc.
1032 Elwell Ct Ste 243
Palo Alto CA 94203
Phone: (650)-968-4470
EMail: robs@join.com
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